Respiratory challenge of H-2^b mice with an H3N2 influenza A virus causes an acute, transient pneumonitis characterized by the massive infiltration of CD8⁺ T lymphocytes. The inflammatory process monitored by quantitative analysis of lymphocyte populations recovered by bronchoalveolar lavage is greatly enhanced by prior exposure to an H1N1 virus, with the recall of cross-reactive CD8⁺-T-cell memory leading to more rapid clearance of the infection from the lungs. The predominant epitope recognized by the influenza virus-specific CD8⁺ set has long been thought to be a nucleoprotein (NP_366–374) presented by H-2D^b (D^bNP₃₆₆). This continues to be true for the secondary H3N2→H1N1 challenge but can no longer be considered the case for the primary response to either virus. Quantitative analysis based on intracellular staining for gamma interferon has shown that the polymerase 2 protein (PA_224–233) provides a previously undetected epitope (D^bPA₂₂₄) that is at least as prominent as D^bNP₃₆₆ during the first 10 days following primary exposure to either the H3N2 or H1N1 virus. The response to D^bNP₃₆₆ seems to continue for longer, even when infectious virus can no longer be detected, but there is no obvious difference in the prevalence of memory T cells specific for D^bNP₃₆₆ and D^bPA₂₂₄. The generalization that the magnitude of the functional memory T-cell pool is a direct consequence of the clonal burst size during the primary response may no longer be useful. Previous CD8⁺-T-cell immunodominance heirarchies defined largely by cytotoxic T-lymphocyte assays may need to be revised.